Condition responsive devices which provide an indication or control a selected operation in response thereto, are well known in the prior art. Such devices have been used in many diverse environments. For example, it is known in the prior art to connect a pressure sensitive element, such as a Bourdon tube, to a source of external pressure, whereby in operation the tube is deflected by an amount proportional to the sensed pressure variations. The mechanical deflection of the Bourdon tube is transduced into a mechanical output to provide an indication of the pressure in the tube or, alternatively, to control a physical operation. An apparatus of this type is shown in U.S. Pat. No. 4,502,334 and in U.S. Pat. No. 3,742,233, the latter describing a transducer for converting the motion of a Bourdon tube into a digital electrical signal.
Although such condition responsive devices have proven generally effective for indicating a condition or controlling an operation in response to a change in the condition, such devices may not be acceptable for providing the necessary degree of safety required in certain atmospheric conditions. For example, the use of the device such as described in U.S. Pat. No. 3,742,233 requires the installation of a light source at the pressure responsive element. This creates a condition for an electrically produced spark which in an explosive atmosphere would have disastrous results. In such applications where there is a potentially explosive environment, even a small electrically produced spark cannot be tolerated due to the potential of a disastrous explosion. Heretofore, compensation to minimize the potential of an explosion due to an electrically produced spark has required expensive and complicated instrumentation with its attendant high cost and potential unreliability.
Accordingly, an advantage of the present invention is to provide a condition responsive apparatus having an encoded mask associated with the pressure sensitive element operating in conjunction with a remote light source and a remote light detector coupled to the mask by means of fiber optic cables.